The mechanisms and regulation of chromosome replication and cell division in E. coli will be examined by studying the timing of plasmid and chromosome replication during the division cycle of E. coli B/r, and temperature-sensitive DNA replication mutants of this organism. The studies on autonomous plasmid replication are designed to determine 1) the timing of replication of F', F and miniF plasmids in relationship to chromosome replication in three substrains of E. coli B/r growing at various rates, and 2) the timing of replication, copy numbers, protein-binding properties and physiological consequences of minichromosomes possessing functional origins of replication. The studies on chromosome replication are designed to determine 1) the involvement of the dnaA, dnaC and dnaI gene products in determining the timing of initiation of chromosome replication, 2) the kinetics of synthesis of DNA-binding proteins with respect to initiation and elongation of the E. coli chromosome, 3) the possibility of diffusable factors produced by dnaA mutants grown at intermediate temperatures capable of enducing entrainment of chromosome replication in growing cultures.